You'll want to use at least two cold sources in an insulated bag to keep perishable foods in your lunch safe; ice or gel packs in your insulated bag or box work best. Perishable foods, such as cold cut sandwiches and yogurt, can be left out at room temperature for no more than 2 hours before they become unsafe to eat.

As a general rule of thumb for food safety in 2026, a sandwich containing "perishable" ingredients (like deli meats, eggs, tuna, or mayonnaise) should not be left out of refrigeration for more than two hours. If the ambient temperature is above 90°F (32°C), such as at a summer picnic or a beach, that safety window drops to only one hour. After this time, bacteria such as Staphylococcus aureus or Salmonella can multiply to dangerous levels that cause food poisoning. If your sandwich is "non-perishable"—such as peanut butter and jelly or a dry veggie sandwich on hearty bread—it can last much longer, often up to 4–6 hours, though the quality will degrade as the bread gets soggy or the fillings wilt. To extend the life of your lunch without an ice pack, you can freeze your juice box or water bottle and place it next to the sandwich; as it thaws, it acts as a temporary cold source. For travel or hiking, it is always safer to stick to shelf-stable ingredients or consume your "fresh" sandwich immediately after purchase or preparation.